The herbicidal activity of 2-(4-aryloxyphenoxy)propionic acids and derivatives thereof is well known in the art. Furthermore, optical isomers are often known to exhibit enhanced herbicidal activity over the corresponding racemates. For example, U.S. Pat. No. 4,531,969 discloses that the R-enantiomers of certain 2-(4-aryloxyphenoxy)propionic acids and certain derivatives thereof are distinguished by a considerably enhanced herbicidal action compared to the racemic modifications. Since reduced quantities of herbicide are required to achieve comparable levels of control, the application of mixtures enriched in the more efficacious R-enantiomer offers both economical and environmental advantages.
In order to commercially exploit the benefits of the advantages associated with the use of the biologically more active enantiomer, it is necessary to accurately determine the ratio of the R- and S-enantiomers in a specific composition. Besides allowing one to monitor the production process, such analytical capability is required to support product labeling and registration requirements. One of the most current approaches to achieving such a resolution and analysis involves the use of chiral mobile-phase additives in liquid chromatography. With this approach, the resolution of enantiomeric compounds can be accomplished by the formation of diastereomeric complexes of the enantiomeric substrate to be resolved and the optically active component added to the eluent. The resolutions are chromatographically effected on the basis of the differences of the diastereomeric complexes formed, e.g., differences in their stabilities, in their solvation in the mobile phase or in their binding to the solid support.
Recently, chiral ion-pairing chromatography has been introduced as a refinement of the above approach. The application of the technique has been extended to enantiomeric carboxylic acids by the addition of naturally-occurring, optically active cinchona alkaloids such as quinine to the mobile phase. The method is based on the formation of diastereomeric amine salts of the carboxylic acid and the optically active cinchona alkaloid.
Although an exceptional resolution of enantiomeric carboxylic acids is possible with this technique, a major drawback is that quinine and related alkaloids are strong ultraviolet absorbers and so provide substantial detector background in the ultraviolet detectors commonly employed in liquid chromatographic analyses. Any slight pressure change, for example, switching an injector valve, upsets the equilibrium causing a baseline excursion. In addition, vacancy chromatography occurs, resulting in inverse peaks, particularly at the retention time for elution of the alkaloid. Consequently, despite exceptional resolution, the procedure is ill-suited to accurate quantitative measurement of enantiomer ratios.